This invention relates to an improved exhaust gas sensor of the type having first and second titania ceramic elements. The titania elements have electrical resistances which vary as a function both of temperature, over the temperature range from about 350.degree. C. to about 850.degree. C., and the partial pressure of oxygen in exhaust gases produced by the combustion of a rich or lean (with respect to stoichiometry) air/fuel mixture.
The first and second titania elements are metal oxide ceramic materials that in use in the exhaust gas sensor are electrically connected in series. The first titania element is more responsive to the partial pressure of oxygen in the exhaust gases than is the second element, but both are responsive to variations in their temperature.
The preferred form of the first titania element is a material which is quite porous, that is, it may have a density of about 2.8 g/cm.sup.3 or less, which is equal to approximately 70% of theoretical density, which is about 4.20 g/cm.sup.3. Of course, the density of the first titania element may be varied to a considerable extent in this porosity range and still retain adequate response characteristics, both with respect to time and resistance. The first titania element, however, does have a resistance when subjected to lean-mixture exhaust gases and at element temperatures within the aforementioned temperature range that are about three orders of magnitude greater than the resistances thereof at corresponding temperatures when subjected to rich-mixture exhaust gases.
In the utilization of the series circuit arrangement for the titania elements in the exhaust gas sensor, it previously was thought that the second titania element or "thermistor" should have a density as close to the theoretical density as possible and, preferably, greater than 97% thereof. It now has been found that considerably improved results are achieved, at least under certain circumstances, if the thermistor or second titania element has a density other than a value approaching its theoretical density.
When the exhaust gas sensor is exposed to temperatures in the upper portion of its operating temperature range, i.e., at temperatures at about 850.degree. C., and when the sensor is exposed for an abnormally long period of time to exhaust gases produced by the combustion of rich air/fuel mixtures, the resistance of the second titania element in the exhaust gas sensor circuit is substantially reduced and approaches that of the first titania element, which is more responsive to the partial pressure of oxygen in the exhaust gases. Thereafter, upon rapid cooling of the exhaust gas sensor, and particularly its second titania element or thermistor, the resistance thereof tends to increase only slightly. Then, if the air/fuel mixture becomes lean so that there is an excess of oxygen in the exhaust gases to which the thermistor is exposed, the thermistor resistance may not change sufficiently to produce the desired exhaust gas sensor output signal representative of the presence of a subsequently occurring rich air/fuel mixture. This can cause errors in the operation of the feedback fuel control system with which the exhaust gas sensor is used in association with the internal combustion engine of a motor vehicle.